Microsoft is putting critical Windows updates on hold after learning that some computers can't start up again after installing the patches.

The updates to Windows included fixes for the Spectre and Meltdown flaws that made a device's central processing unit vulnerable to hacking attacks. The PCs that couldn't reboot had processors made by AMD, one of the three chipmakers whose products were affected by Spectre.

"Microsoft is working with AMD to resolve this issue and resume Windows OS security updates to the affected AMD devices via Windows Update and WSUS as soon as possible," the company said.

In a statement, AMD also said it's collaborating with Microsoft to provide a working update as soon as possible. "AMD is aware of an issue with some older generation processors following installation of a Microsoft security update that was published over the weekend," the company said.

The halted update adds to the chaos surrounding the revelation of the chip flaws, which could let hackers access secret information from processors on computers, phones and servers. The news of the flaws broke on Wednesday, before AMD, Intel and ARM had finished coordinating their response to the research that found the flaws.

Hundreds of millions of devices contain affected chips, and the chipmakers worked with companies like Windows and Apple to update operating systems, as well as with cloud service providers like Google and Amazon to patch potentially affected servers.